Referring to FIG. 18, an oscillator circuit may include inverters 100, 101, 102 connected in series and in which the output of the inverter 101 and the input of the inverter 100 are connected via a capacitor 103. The output of the inverter 102 and the input of the inverter 100 are connected via a resistor 104. FIG. 19 shows waveforms of the input of the inverter 100 (A), the output of the inverter 100 (B), the output of the inverter 102 (Fout), and the output of the inverter 101 (C) at the room temperature, as the solid line.
The oscillation frequency f1 may be determined by the time constant of the charge/discharge of resistor 104 and capacitor 103, and the value of f1 may be given by the following equation for the resistance R11 of the resistor 104 and the capacitance C11 of the capacitor 103:
f1=1/(k·R11·C11), in which k is a constant have a value of approximately 2.2.
When the oscillator is composed of semiconductor elements, the resistor 104 may be made from a diffused resistor or a polysilicon resistor, and the capacitor 103 may be made from a polysilicon interlayer film or a gate oxide film.
The temperature dependency (temperature characteristics) of the resistor 104 may be the primary factor affecting the temperature dependency (temperature characteristics) with respect to the oscillation frequency for the time constant of the charging/discharging determined by the resistor 104 and the capacitor 103. The value may vary in the range of 10 to 40% when using the diffusion resistor (in a ratio of 125° C./room temperature) or in the range of 4 to 10% when using the polysilicon resistor. More specifically, when using a resistor 104 having a smaller temperature coefficient and an interlayer film capacitor as the capacitor 103, displacement will occur at temperatures lower or higher than room temperature. This is illustrated in FIG. 19 by the dotted line and dashed line respectively. The oscillation frequency will have the temperature dependency due to the temperature dependency (temperature characteristics) of the resistor.
When a system requires a higher precision oscillation frequency, an oscillator such as crystals and ceramics is used. However the materials and implementation of such external parts will result in an inevitable augmentation of manufacturing cost.
A circuit design as shown in FIG. 20 is conceivable for stably controlling a frequency on a semiconductor chip. In this circuit design, one of the input terminals of the comparator 110 is fed back through a CR circuit of a resistor 111 and a capacitor 112. The other input terminal of the comparator 110 is connected to a node in a resistor 113 (dividing node) through a first group of switches 114 and a first switch 115, and is also connected to another node in a resistor 113 (dividing node) through a second group of switches 116 and a second switch 117. The first and second switches 115 and 117 are alternately turned on and off in response to the output from the comparator 110. The circuit design also includes a thermistor 118 and a memory 119. The memory 119 will turn on a predetermined switch among the first and second groups of switches 114 and 116 in accordance with the result of a temperature measured by the thermistor 118. More particularly, the memory 119 will selectively turn on any one of the switches among the first group of switches 114, and any one of the switches among the second group of switches 116. The input terminal of the comparator 110 will be applied with an appropriate threshold voltage corresponding to the temperature. Generally, based on the output of the thermistor 118 and the signals from the appropriately preprogrammed memory 119, the threshold voltage of the comparator 110 that determines the oscillation frequency will be adjusted to control the frequency so that it is stable.
However, this circuit design has the drawback of a control circuit section requiring a large area, which leads to an increase in associated cost.